Aid group Doctors Without Borders says 21 people were injured in the attack.

A soldier holds a machine gun mounted on a police truck outside Yemen's parliament during the first parliament session since civil war began almost two years ago, in Sanaa, Yemen, on Saturday.

DUBAI (Reuters) - At least 10 children were killed and 21 injured in northern Yemen on Saturday, aid group Medecins Sans Frontieres said, in what the country’s dominant Houthi group said was a Saudi-led air strike on a school.

The tragedy occurred as Yemen’s parliament convened for the first time in almost two years, in a move that bolsters the Houthi movement and challenges the Saudi-backed exiled government.

A spokesman for the Saudi-led coalition had no comment on the attack. Saudi Arabia and its allies have launched thousands of air strikes against the Houthis since they drove the internationally recognized government into exile in March 2015.

“@MSF received today 21 injured & 10 deaths in Haydan #Saada. All were under 15 years old,”said the medical aid group, also known as Doctors Without Borders, on its official Twitter account.

Mohammed Abdul-Salam, a senior official from the Houthi movement whose main support base is in Saada, blamed the attack on the mostly Gulf Arab military coalition that has been at war with the group for over a year.

At least 6,400 people have been killed in the conflict, around half of them civilians.

The armed Houthis and their allies in the General People’s Congress (GPC) party headed by powerful ex-president Ali Abdullah Saleh control the capital Sanaa and have so far withstood the coalition campaign.

President Abd-Rabbu Mansour Hadi and his internationally recognized government view them as putschists after they took the capital by force in September 2014 and have warned them against seeking to legitimize their rule through parliament.

In a statement carried on the state news agency Saba, Hadi called the parliament session illegal and warned that MPs attending it could be prosecuted as criminals.

After U.N.-backed peace talks to end the war collapsed last week, the Houthis and the GPC set up a governing council to rule the country despite U.N. and government opposition.

The assembly convened within earshot of bombings by Saudi-led warplanes on military bases several miles away